EP2419695B1

EP2419695B1 - Displacement sensor

Info

Publication number: EP2419695B1
Application number: EP10764581.4A
Authority: EP
Inventors: Chul Sub Lee; Eul Chul Byeon; Beom Kyu Kim; Young Deok Kim
Original assignee: Tyco Electronics AMP Korea Co Ltd
Current assignee: Tyco Electronics AMP Korea Co Ltd
Priority date: 2009-04-14
Filing date: 2010-03-09
Publication date: 2018-01-24
Anticipated expiration: 2030-03-09
Also published as: US8960024B2; EP2419695A4; EP2419695A2; WO2010120042A3; CN102395857B; KR20100113883A; US20120011942A1; WO2010120042A2; CN102395857A; KR101637040B1

Description

Technical Field

The present invention relates to a displacement sensor to measure displacement of a variety of devices depending on a slight deformation of the devices, and more particularly, to a displacement sensor to measure displacement of a device based on moment of force acting on a diaphragm when the diaphragm undergoes displacement.

Background Art

An example of a conventional displacement sensor is illustrated in FIG. 7.
The illustrated conventional displacement sensor 300 includes a body 310 mounted to a mechanical device D1, an input bar 320 in the form of a cantilever integrally extending from the body 310, a distal end of which comes into contact with a partial region D2 of the mechanical device D1 so as to undergo displacement according to displacement of the partial region D2, a strain gauge 331 attached to the input bar 320, and an electric circuit board 332 to generate an electrical signal upon receiving a strain value measured by the strain gauge 331.
When the partial region D2 is moved upward, the input bar 320 is bent upward thus undergoing displacement. The strain gauge 331 and the electric circuit board 332 generate an electrical signal corresponding to the displacement of the input bar 320.
In the above described conventional displacement sensor 300, the strain gauge 331 is attached to the input bar 320 to attain a sufficient sensitivity. In addition, to prevent contamination due to impurities under specific use environments, a cover 340 is provided to maintain the strain gauge 331 and the electric circuit board 332 in an airtight state.
In this case, to maintain the strain gauge 331 in an airtight state, it is necessary to attach a part of the cover 340 to the input bar 320. However, this causes the cover 340 to undergo displacement along with the input bar 320, having an effect on a measured value of the strain gauge 331.
Moreover, the cover 340 may be unintentionally detached from the input bar 320 due to frequent deformation of the input bar 320.
FIG. 8 is a partial sectional view illustrating a conventional load sensor using a diaphragm, to which a strain gauge is attached.
The conventional load sensor 200 includes a sensing body 210, a diaphragm 211 provided on the sensing body 210, and an input bar 212 provided beneath the sensing body 210 to vertically press the diaphragm 211 by a weight thereof.
A strain gauge 213 is attached to the diaphragm 211. The strain gauge 213 includes a pair of first and second piezo-resistance elements R₁ and R₂ attached close to one side edge of the diaphragm 211, and a pair of third and fourth piezo-resistance elements R₃ and R₄ attached close to an opposite side edge of the diaphragm 211 so as to correspond respectively to the first and second piezo-resistance elements R₁ and R₂.
A notch 214 is indented in a lower surface of the sensing body 210 and acts to increase strain of the diaphragm 211.
Assuming that the input bar 212 is fixed at or comes into contact with a load occurrence position of a target device to be measured, the input bar 212 is mainly subjected to vertical force. In addition to the vertical force, the input bar 212 is typically subjected to horizontal force, twisting moment, etc, thus being under the influence of miscellaneous load including moment, torsion, etc.
When attempting to detect deformation of the diaphragm 211 caused by the vertical force to be measured by means of the strain gauge 213, deformation of the diaphragm 211 due to the miscellaneous load including moment, etc. may be detected simultaneously. Therefore, it is necessary to eliminate the miscellaneous load in order to measure only the vertical force.
Accordingly, in a conventional solution, as shown in FIG. 9, a Wheatstone bridge circuit consisting of first to fourth piezo-resistance elements is used. The Wheatstone bridge circuit is configured such that strains measured by the piezo-resistance elements under the influence of moment offset each other and only deformation of the diaphragm caused by vertical force can be measured.
In the meantime, in the case where the conventional load sensor is used as a displacement sensor, it is necessary for the diaphragm to be oriented orthogonal to a surface of a mechanical device that undergoes displacement. This disadvantageously results in a limited installation position of the sensor.
In particular, if a possible installation space of the displacement sensor is limited, for example, if it is difficult, in the case of measurement of displacement of a vehicular electronic brake caliper, to attain a space required for the displacement sensor to be orthogonally attached to a displacement occurrence surface, the use of the displacement sensor may be impossible.
With relation to design of a sensor, it is important to provide the sensor with not only high strength, but also sufficient strain for stable amplification in a circuit. However, the sufficient strain and the high strength are conflicting characteristics from various viewpoints and thus, design trade-off is necessary. For this reason, when a displacement sensor is designed based on the conception of a load sensor that is adapted to receive force directly, the displacement sensor may entail a problematic strength, resulting in vulnerable sensor design. Accordingly, to enable stable measurement of displacement regardless of a maximum operating load, it is necessary to design a displacement sensor such that the role of the displacement sensor is limited to accurately measure slight displacement of a specific region of a structure and a system operating load is assigned to the structure. This is a principal aim of design of the displacement sensor. This is also advantageous for acquisition of a sensor installation space because it is unnecessary to arrange the sensor on a transmission path of force. Accordingly, upon design of the sensor, a system designer should consider only an operational displacement portion of the structure that can be measured by the sensor.
In the meantime, the above described conventional diaphragm has a significantly limited strain gauge attachment area. This results in troublesome attachment of the strain gauge and increases generation of defective products due to a deviated attachment position of the strain gauge.
More specifically, to accurately measure strain using the strain gauge, the strain gauge must be attached to a linearly deformable region of the diaphragm. In the case of the above described conventional diaphragm, only a partial region immediately above the notch undergoes approximate linear deformation and thus, the strain gauge must be accurately attached to the partial region. Therefore, despite use of an automated machine, there always exists a risk of a deviated attachment position of the strain gauge due to fine shaking and thus, generation of defective products may be increased.
US 5263375 discloses a load sensor comprising a sensing body including a diaphragm provided at a lower surface thereof. A strain gauge is formed on a lower surface of the sensing body and includes four piezo-resistance elements. An input portion is orthogonally fixed at the center of an upper surface of the sensing body.
DE 102005036126 , DE 2608381 and JP 2005 106775 disclose similar load sensors. DE3238951 discloses a load sensor wherein the strain gauges are on the same side of the diaphragm as the input portion.

Disclosure of Invention

Technical Problem

Therefore, the present invention has been made in view of the above problems, and it is one object of the present invention to provide a displacement sensor, which enables measurement of displacement caused by moment even with use of a diaphragm to which a strain gauge is attached.
It is another object of the present invention to provide a displacement sensor in which a Wheatstone bridge circuit is adapted to measure only deformation caused by moment while eliminating strain caused by vertical force.
It is another object of the present invention to provide a displacement sensor in which an input bar converts displacement orthogonal to a plane of a diaphragm into moment, thereby enabling deformation of the diaphragm.
It is another object of the present invention to provide a displacement sensor in which a diaphragm has an increased linearly deformable area where a strain gauge is attachable.
It is a further object of the present invention to provide a displacement sensor in which deformation of a diaphragm can be amplified, resulting in enhanced sensitivity.

Technical Solution

In accordance with the present invention, the above and other objects can be accomplished by the provision of a displacement sensor according to claim 1.
The sensing body may include an annular notch indented in the upper surface thereof, and a slope extending obliquely from the annular notch toward the center of the upper surface thereof to increase a linearly deformable area of the diaphragm.
The diaphragm may include an attachment surface to which the strain gauge is attached and a disc-shaped protrusion having the same center as the center of the diaphragm and serving to increase strain of the attachment surface.
The diaphragm may include an attachment surface to which the strain gauge is attached and a disc-shaped protrusion having the same center as the center of the diaphragm and serving to increase strain of the attachment surface.
The displacement sensor may further include a housing perforated with an opening, into which the diaphragm is inserted and coupled, the housing encasing the signal processing unit therein, and a connector provided at a side surface of the housing while being connected to the signal processing unit to transmit the electrical signal to the outside.

Advantageous Effects

As apparent from the above description, a displacement sensor according to the embodiment of the present invention has the following several effects.
Firstly, the displacement sensor can measure displacement caused by moment even with use of a diaphragm to which a strain gauge is attached. Accordingly, the displacement sensor has no limit in an attachment position thereof.
Secondly, the strain gauge is attached to the diaphragm in a direction parallel to a moment generation direction, so as to measure maximum strain of the diaphragm caused by moment. As a result, the displacement sensor can attain a strain value suitable for amplification by a circuit while maintaining an appropriate strength thereof.
Thirdly, with use of a signal processing unit adopting a Wheatstone bridge circuit, vertical force applied to the diaphragm simultaneously with moment can be eliminated and thus, can be excluded from the subject of measurement. This enables more accurate calculation of displacement caused by moment.
Fourthly, the diaphragm has a slope to increase a linearly deformable area thereof. The increased linearly deformable area has the effect of increasing a tolerance range of an attachment position of the strain gauge, thus reducing generation of defective products due to incorrect attachment of the strain gauge.
Fifthly, when the diaphragm is centrally provided with a disc-shaped protrusion, a peripheral region of the diaphragm, to which the strain gauge is attached, is subjected to greater strain, resulting in enhanced sensitivity.

Brief Description of Drawings

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view illustrating a displacement sensor according to an embodiment of the present invention;
FIG. 2 is an exploded bottom perspective view of the displacement sensor illustrated in FIG. 1;
FIG. 3 is a perspective view illustrating a diaphragm and an input bar provided in the displacement sensor illustrated in FIG. 1;
FIG. 4 is a sectional view of the diaphragm and the input bar illustrated in FIG. 3;
FIG. 5 is a schematic diagram of a Wheatstone bridge circuit provided in the displacement sensor illustrated in FIG. 1;
FIG. 6 is a sectional view of the diaphragm provided in the displacement sensor illustrated in FIG. 1;
FIG. 7 is a sectional view illustrating a conventional displacement sensor;
FIG. 8 is a sectional view illustrating a conventional load sensor; and
FIG. 9 is a schematic diagram of a Wheatstone bridge circuit provided in the load sensor illustrated in FIG. 8.

Best Mode for Carrying out the Invention

Hereinafter, functions, configurations and operations of a displacement sensor according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a perspective view illustrating a displacement sensor according to an embodiment of the present invention, and FIG. 2 is an exploded bottom perspective view of the displacement sensor illustrated in FIG. 1.
The displacement sensor 100 according to the embodiment of the present invention includes a sensing body 1 and a signal processing unit 2. The sensing body 1 is provided at a lower surface thereof with a diaphragm 11 to which a strain gauge 12 is attached, and an input bar 13 orthogonally fixed to the center of an upper surface thereof, to which displacement of a mechanical device is transmitted. The strain gauge 12 serves to measure strain of the diaphragm 11 caused by moment of the input bar 13, and the signal processing unit 2 generates an electrical signal based on a strain value output from the strain gauge 12.
The displacement sensor 100 may further include a housing 3 in which the diaphragm 11 of the sensing body 1 and the signal processing unit 2 are accommodated, and a connector 4 to receive the electrical signal generated by the signal processing unit 2. The housing 3 has an opening 31 for coupling of the diaphragm 11, and the signal processing unit 2 is encased within the housing 3 so as not to be contaminated by impurities.
The housing 3 may function to shield electromagnetic waves generated from other electronic devices, and may further have a flange 32 having screw holes to install the displacement sensor 100 to the mechanical device.
A cover 3a is attached to the housing 3 by welding in a final assembly stage. The cover 3a serves as a shield to improve Electro Magnetic Compatibility (EMC) characteristics and enables realization of a waterproof sensor. In addition, attachment of the cover 3a provides the housing 3 of the displacement sensor 100 with a closed box-shaped structure. With this configuration, when the displacement sensor 100 is mounted to a target object, i.e. the mechanical device, it is possible to prevent deformation of the flange 32, caused by, e.g., bolt tightening or flatness of a mounting surface of the mechanical device, from having an effect on the sensing body 1 and consequently, to reduce a mounting offset error, an important characteristic of the displacement sensor 100.
The input bar 13 has an L-shaped form consisting of an upper portion parallel to a horizontal plane of the diaphragm 11 and a lower portion orthogonal to the horizontal plane of the diaphragm 11. When a transmission member T, which constitutes a part of the mechanical device and comes into contact with one end of the input bar 13, is moved upward, the other end of the input bar 13 fixed to the sensing body 1 undergoes moment. The moment causes deformation of the diaphragm 11 of the sensing body 1 and the strain gauge 12 measures strain of the diaphragm 11.
FIG. 3 is a perspective view illustrating the diaphragm and the input bar provided in the displacement sensor illustrated in FIG. 1, and FIG. 4 is a sectional view of the diaphragm and the input bar illustrated in FIG. 3.
The strain gauge 12 attached to the diaphragm 11 is arranged parallel to moment generation direction by the input bar 13. The strain gauge 12 includes first to fourth piezo-resistance elements R₁, R₂, R₃ and R₄, which are sequentially attached to an imaginary line passing through the center of the diaphragm 11 such that the first and second piezo-resistance elements R₁ and R₂ are symmetrical respectively to the third and fourth piezo-resistance elements R₃ and R₄.
In FIG. 1, a longitudinal direction of the L-shaped input bar 13 is referred to as an X-axis. Also, a Y-axis is orthogonal to the X-axis and is parallel to the plane of the diaphragm 11, and a Z-axis is orthogonal to the X-axis and Y-axis directions.
Assuming that the transmission member T is moved upward in the Z-axis direction, the diaphragm 11 undergoes moment M1 about a rotation axis, i.e. the Y-axis in an X-Z plane. Accordingly, as illustrated in FIG. 3, the piezo-resistance elements R₁, R₂, R₃ and R₄ of the strain gauge 12 are attached parallel to one another in an X-axis direction in which the diaphragm 11 undergoes maximum deformation under the influence of the moment M1. In this case, the strain gauge 12 can measure maximum strain of the diaphragm 11 caused by the moment M1, resulting in an accurate measured value.
In FIG. 4, when displacement of the transmission member T is transmitted such that the distal end of the input bar 13 is moved upward, moment is generated causing the first piezo-resistance element R₁ and the third piezo-resistance element R₃ to be tensioned (lengthened) and the second piezo-resistance element R₂ and the fourth piezo-resistance element R₄ to be compressed (shortened). On the contrary, when the displacement is transmitted such that the distal end of the input bar 13 is moved downward, moment is generated in an opposite direction thus causing the first piezo-resistance element R₁ and the third piezo-resistance element R₃ to be compressed (shortened) and the second piezo-resistance element R₂ and the fourth piezo-resistance element R₄ to be tensioned (lengthened) (See the following Table 1).
The signal processing unit 2 includes a Wheatstone bridge circuit as illustrated in FIG. 5, and generates an electrical signal based on the above described displacement as represented in the following Equation 1.
$\begin{array}{l} \frac{ΔV}{V} = \frac{R_{1} R_{3} - R_{2} R_{4}}{(R_{1} + R_{2}) (R_{3} + R_{4})}, \\ \frac{ΔV}{V} = \frac{K}{4} (ε_{1} - ε_{2} + ε_{3} - ε_{4}) \end{array}$
Here, R₁, R₂, R₃ and R₄ are resistance values corresponding to the respective piezo-resistance elements, K is a fixed proportional constant value representing a gauge factor, and ε₁, ε₂, ε₃, and ε₄ are strain values measured by the respective piezo-resistance elements.
The Wheatstone bridge circuit having the above described configuration generates an electrical signal having a positive (+) value calculated by the above Equation 1 when the displacement is transmitted such that the distal end of the input bar 13 is moved upward, but generates an electrical signal having a negative (-) value when the displacement is transmitted such that the distal end of the input bar 13 is moved downward.
In the meantime, the Wheatstone bridge circuit according to the present invention eliminates vertical force which is applied to the diaphragm 11 along with the moment. That is, the Wheatstone bridge circuit is operated to eliminate vertical force while amplifying moment. The moment amplifying displacement sensor deals with vertical force as one of miscellaneous load.
More specifically, when the displacement is transmitted such that the distal end of the input bar 13 is moved upward, there occurs force F1 to vertically raise the center of the diaphragm 11.
In this case, the first piezo-resistance element R₁ and the fourth piezo-resistance element R₄ are tensioned (lengthened) and the second piezo-resistance element R₂ and the third piezo-resistance element R₃ are compressed (shortened). In addition, it can be appreciated that the electrical signal calculated by the above Equation 1 has zero value because strain values of the symmetrically attached first and third piezo-resistance element R₁ and R₃ and strain values of the symmetrically attached second and fourth piezo-resistance element R₂ and R₄ have the same magnitude.
Even when the displacement is transmitted such that the distal end of the input bar 13 is moved downward, the sum of strain values of the first to fourth piezo-resistance elements may be zero and thus, the electrical signal calculated by the above Equation 1 has zero value.
The above described principle is summarized in the following Table 1.

[Table1]

[Table]

	ε₁	ε₂	ε₃	ε₄	Electrical Signal
First Case(+ moment)	+ value	- value	+ value	- value	+ value
Second Case(-moment)	- value	+ value	- value	+ value	- value
Third Case (Vertical Compression)	+ value	- value	- value	+ value	Zero value
Fourth Case(Vertical Tension)	- value	+ value	+ value	- value	Zero value

Here, the first case represents strain caused by moment when the displacement is transmitted such that the distal end of the input bar is moved upward, the second case represents strain caused by moment when the displacement is transmitted such that the distal end of the input bar is moved downward, the third case represents strain caused when vertical compression force is applied to the input bar, and the fourth case represents strain caused when vertical tensile force is applied to the input bar.
As described above, the electrical signal generated by the signal processing unit using the Wheatstone bridge circuit has a positive (+) value or a negative (-) value, and the magnitude of input displacement is calculated from the magnitude of the electrical signal. In addition, the Wheatstone bridge circuit eliminates force acing in a vertical direction of the diaphragm, enabling accurate calculation of displacement caused by moment.
FIG. 6 is a sectional view of the diaphragm provided in the displacement sensor illustrated in FIG. 1.
The sensing body 1 according to the present invention has an annular notch 1a indented in the upper surface thereof to amplify deformation of the diaphragm 11. In addition, the sensing body 1 has a slope 1b obliquely extending from the notch 1a toward the center of the upper surface thereof, the slope 1b serving to increase a linearly deformable area of the diaphragm 11. Specifically, the slope 1b extends by a predetermined inclination angle inward from the bottom of the notch 1a indented in the upper surface of the sensing body 1.
As the slope 1b is inclined by a predetermined angle with respect to the horizontal plane of the diaphragm 11, the diaphragm 11 undergoes linear deformation in the slope 1b.
This principle is known and is equal to that of a cantilever, which has a downwardly inclined upper surface and a horizontal lower surface, and is linearly deformed when downward shear stress is applied to a distal end of the cantilever.
In this case, as the linearly deformable area of the diaphragm 11 where the strain gauge 12 is attachable increases, a tolerance range of an attachment position of the strain gauge 12 can be increased. This can reduce generation of defective products due to incorrect attachment of the strain gauge 12.
In the meantime, to enhance sensitivity of the displacement sensor 100 by amplifying detected values of the respective piezo-resistance elements R₁, R₂, R₃ and R₄ of the strain gauge 12, preferably, the diaphragm 11 is further provided with a disc-shaped protrusion 111 having the same center as that of the diaphragm 11.
With provision of the disc-shaped protrusion 111, the center region of the diaphragm 11 has a thickness greater than a thickness of the remaining region of the diaphragm 11. Consequently, the center region of the diaphragm 11 having the disc-shaped protrusion 111 is subjected to a smaller strain, whereas a peripheral region of the diaphragm 11 to which the strain gauge is attached, i.e. an attachment surface 112 of the diaphragm 11 is subjected to a greater strain.
In this case, when the attachment surface 112 is subjected to the greater strain, the displacement sensor 100 can perform more accurate measurement even slight moment variation with high sensitivity.

Mode for the Invention

Various embodiments have been described in the best mode for carrying out the invention.

Industrial Applicability

The present invention is applicable to a displacment sensor to measure displacement caused by moment even with use of a diaphragm to which a strain gauge is attached.
Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the invention as disclosed in the accompanying claims.

Claims

A displacement sensor (100) comprising:
a sensing body (1) including a diaphragm (11) provided at a lower surface thereof, to which a strain gauge (12) is attached, and an input bar (13) orthogonally fixed at the center of an upper surface thereof, to which displacement is transmitted, the strain gauge (12) being attached to an imaginary line that is parallel to a direction in which the diaphragm (11) undergoes maximum deformation under the influence of a moment (M1) applied to the diaphragm (11) when displacement is transmitted to the input bar (13) that passes through the center of the diaphragm (11), the strain gauge (12) including a first piezo-resistance element (R₁), a second piezo-resistance element (R₂), a third piezo-resistance element (R₃) and a fourth piezo-resistance element (R₄) attached in sequence, the first and second piezo-resistance elements (R₁, R₂) being symmetrical respectively to the third and fourth piezo-resistance elements (R₃, R₄), the input bar (13) having an L-shaped form consisting of an upper portion parallel to a plane of the diaphragm (11) and a lower portion orthogonal to the plane of the diaphragm (11); and

a signal processing unit (2) to generate an electrical signal based on an output value of the strain gauge (12) corresponding to strain of the diaphragm (11) caused by only moment of the input bar (13), the signal processing unit (2) generating an electrical signal corresponding to strain of the diaphragm (11) by use of the following Equation; $\frac{ΔV}{V} = \frac{K}{4} (ε_{1} - ε_{2} + ε_{3} - ε_{4})$
with K being a proportional constant, and ε₁ to ε₄ being strain values measured by the first to fourth piezo-resistance elements.
The displacement sensor according to claim 1, wherein the sensing body (1) includes an annular notch (1a) indented in the upper surface thereof, and a slope (1b) extending obliquely from the annular notch (1a) toward the center of the upper surface thereof to increase a linearly deformable area of the diaphragm (11).
The displacement sensor according to claim 1, wherein the diaphragm (11) includes an attachment surface (112) to which the strain gauge (12) is attached and a disc-shaped protrusion (111) having the same center as the center of the diaphragm (11) and serving to increase strain of the attachment surface (112).
The displacement sensor according to claim 3, further comprising: a housing (3) perforated with an opening (31), into which the diaphragm (11) is inserted and coupled, the housing (3) encasing the signal processing unit (2) therein; and a connector (4) provided at a side surface of the housing (3) while being connected to the signal processing unit (2) to transmit the electrical signal to the outside.